Television systems in which the picture tubes (CRT's) project their images upon a remote screen produce a greatly enlarged picture. Because of brightness problems encountered when a single three-color picture tube is used, color TV systems of the projection type commonly employ three different monochrome picture tubes, one for each of the three primary colors. But the three-tube design raises the question of how to achieve registration of the three different color images upon the projection screen.
One way of doing this is to use dichroic prisms to fold the light output of the three picture tubes into a common optical path. While this approach provides good image registration, the dichroic prisms reduce light transmission to levels generally considered unacceptable. The preferred approach to image registration is to arrange the three picture tubes side-by-side (the "in-line" configuration), with one of the tubes located on the axis of the projection screen and the other two located off-axis on either side thereof. The respective focussing lenses of the off-axis picture tubes are then skewed so that their optical axes converge on a central projection screen location. This arrangement produces a bright picture, but it directly or indirectly causes a number of problems which it is the object of this invention to solve or minimize.
One of these problems has to do with the coincidence of the three focal planes. Because of the skewed orientation of the off-axis focussing lenses, their projected images have focal planes which do not coincide with the projection screen. Instead, these focal planes are rotated so that they diverge from the screen at the lateral edges thereof.
In the past, system designers have used relatively slow f/2, lenses, and depended upon the resulting depth of focus to prevent this divergence from being noticeable. But recently they have experimented with faster f/1 lenses, which yield a brighter picture but have less depth of focus. With such lenses the amount of defocussing at the lateral edges of the screen is noticeable.
In accordance with one aspect of this invention, this defocussing problem is dealt with by rotating each off-axis picture tube image relative to its focussing lens, until its projected image becomes congruent with the projection screen surface.
U.S. Pat. No. 4,194,216 of Ohmari shows a partially in-line color projection TV system in which the picture tubes are skewed with respect to the optical axes of the lenses. But the Ohmari system employs parallel-axis focussing lens assemblies; and such a system does not have the problem of skewed focal planes with which this invention is concerned. Ohmari rotates the picture tubes with respect to the optical axes of the lenses for an entirely different purpose; i.e., to compensate for trapezoidal distortion of the projected images.
Another problem which results from the off-axis placement of some of the picture tubes in an in-line projection system is color shading at the extremes of the image on the projection screen. One cause of color shading is inherent in the off-axis position of two of the three image projectors. Whenever the axis of image projection is at an angle to the projection screen, a portion of the projected image which is on one side of the projection axis travels farther, and a portion which is on the opposite side of the axis travels less, than the axial distance from the image source to the projection screen. The half of each image which travels farther will therefore be darker than the other half, due to the fact that the intensity of illumination must fall off as the square of the distance from the light source. Since the two off-axis projectors are on opposite sides of the projection screen axis, the half of the composite image which is deficient in one of the off-axis colors will have an excess of the other off-axis color, and vice versa. Thus the two halves of the composite image will have opposite color imbalances.
A paper entitled "Developments in Plastic Optics for Projection Television Systems," by Roger L. Howe and Brian H. Welham, which was presented on Nov. 13, 1979 to the I.E.E.E. Chicago Fall Conference on Consumer Electronics, and which has been published by U.S. Precision Lens Incorporated, recognizes (on page 8) that this color shift is proportional to the angular separation of the lenses.
In accordance with another aspect of this invention, the effect of uneven square law fall-off can be reduced by designing the system so that the angular deviation of the off-axis projectors from the projection screen axis is held to the minimum which is compatible with manufacturing tolerances and other design requirements.
Color shading also results as an undesirable by-product of a technique which is used to compensate for trapezoidal distortion. The off-axis images tend to be trapezoidally distorted (i.e., rectangles on the CRT screen tend to appear as trapezoids on the projection screen) owing to the fact that so far as the off-axis images are concerned, the projection screen is not perpendicular to the axis of image projection.
Such trapezoidal (or keystone) distortion can be minimized, but not eliminated, by clustering the picture tubes closely together as suggested in Oland U.S. Pat. No. 4,004,093. Or it can be compensated by laterally offsetting the picture tubes from their focussing lenses, as disclosed in the Ohmari patent mentioned above and also in Nishimura U.S. Pat. No. 4,087,835. The preferred method of compensation for trapezoidal distortion, however, is by trapezoidally pre-distorting the image on the CRT in the opposite sense, which restores the desired rectangular shape on the projection screen.
There are various techniques for accomplishing such compensatory pre-distortion. For example, it can be done electrically by varying the sweep signal amplitude as suggested in Koyama U.S. Pat. No. 3,949,167, or magnetically by altering the shape of the deflection coil field as suggested in Marley U.S. Pat. No. 3,115,544. But the preferred pre-distortion technique is to employ at each off-axis position a picture tube the electron gun of which is slanted relative to the tube's faceplate and phosphor screen. This approach is described in U.S. Pat. No. 4,274,110 of Stanley Lehnert, entitled "Projection Television System". The slanted electron gun produces the desired trapezoidal predistortion; but it also promotes color shading, for the following reasons.
It is an inherent property of the lenses which are used to focus the projected images that the illumination reaching the projection screen from each lens is highest at the optical axis of the lens, and falls off at any distance from that axis as the fourth power of the cosine of the angle of deviation therefrom, an effect known as optical vignetting. Illumination can also fall off abruptly at the extremes of the projected image as a result of mechanical vignetting. This can occur when some of the lens elements are not large enough. Manufacturing variations in the thickness of the CRT phosphor screen and of the aluminization layer on the CRT faceplate also contribute to vignetting. Because of the way it is made, the phosphor layer tends to be thinner, and therefore to emit less light, at the edge of the CRT screen than in the middle. Furthermore, the oblique angle of incidence of the electrons on the phosphor screen nearer the edges thereof reduces the depth of penetration of the electrons. In addition, reflection from the lens elements causes greater light losses at the extremes of the image than it does in the center, because of the less favorable angle of incidence. For all these reasons, all three of the different colored images are brighter in the center of the projection screen than they are at the edges thereof. Moreover, this effect is more pronounced in the larger aperture (e.g. f/1) lenses, which are coming into greater use.
If it were not for the trapezoidally pre-distorted rasters, all three of the colored images would exhibit the same variation of brightness as a function of position on the projection screen, and therefore no change in color balance would occur across the projected image. But such pre-distortion of the off-axis images has the effect of increasing the displacement from the center of the CRT raster of each image point on the enlarged end of the trapezoid and decreasing that displacement as to each image point on the reduced end of the trapezoid. These increases and decreases in image point displacement on the CRT screen then (because of the angular brightness variations introduced by the various sources of vignetting described above) are translated into differences in brightness on the projection screen. Image points on the enlarged side of the trapezoid (as it appears on the CRT) are reduced in brightness during passage through the projection lens because of their increased displacement from the lens axis, while image points on the reduced side of the trapezoid are increased in brightness for the converse reason. Thus, upon emerging from their projection lenses the off-axis images suffer from a brightness gradient extending from one edge to the opposite edge. When these images fall on the projection screen, the trapezoidal pre-distortion is corrected but the brightness gradient is not.
Moreover, the brightness gradients of the two off-axis images are oppositely directed, because their CRT pictures are trapezoidally distorted in opposite directions. Therefore each one of the off-axis colored images increases in brightness toward that edge of the projection screen where the other off-axis colored image decreases. The result is a color gradient which produces one tint at one edge and the opposite tint at the opposite edge. True color balance between the two off-axis colored images occurs only in the center of the projection screen. In a typical projection TV system employing an f/1 lens, variations of as much as 10% in light transmission can occur at the image edges, producing noticeable color imbalances.
In accordance with another aspect of this invention, the color shading problem is dealt with by offsetting the object image on the CRT face from the optical axis of its lens assembly. This distorts the CRT image as seen from the lens assembly and, when the offset is in the proper direction, is equivalent to moving the image points in the large end of the trapezoid closer to the optical axis and the image points in the small end further away, thus correcting the brightness bias which results from the previously unequal distances of these two sets of image points from the optical axis.
In the Ohmari and Nishimura patents cited above, the picture tubes are laterally displaced relative to their focussing lens optical axes, but again this is done for an entirely different purpose: i.e. to reduce the trapezoidal distortion.
These and other features of the invention will be more fully appreciated from the following detailed description, when read in conjunction with the accompanying drawings.